Hm, one door is opened via lever (10), leading eventually to Samuel (11)
One door is open (8), after you have tricked Kegan out of his key (4)
one door is open (7)after you have received imp Rashax's storage key (6).
Door at 13 it open after you have received the key in Samuel's lair (12) - same goes for the locked door at 14 - you find the key in his lair, too.
http://beyond.divinedivinity.net/help/act1_citadel10.htmany locked door I missed?